1. Field of the Invention
The present invention relates generally to the field of voltage detection devices, particularly to power-on reset (POR) circuits.
2. Description of Background Art
A power supply provides voltage signals to digital or analog integrated circuits (IC's). In order for a digital IC to operate properly the supply voltages generated by the power supply must exceed a minimum voltage. In general, the supply voltages supplied to a digital or analog IC are greater than or equal to the voltage used by the internal logic of the IC to differentiate between a binary one and a binary zero. Conventional IC have operating voltages of 5.0 volts. More recent IC have operating voltages of 3.3 volts, 2.4 volts, or 2.0 volts.
A power supply generates a supply voltage upon turn-on that increases (ramps) from zero volts to the operating voltage of the IC. Conventional power supplies may require from a few milliseconds to several hundred milliseconds to generate a supply voltage equal to the operating voltage, i.e., to "power-up" to the operating voltage. While a power supply coupled to an IC is powering-up, the internal logic within the IC is in an unknown state. That is, the value of the logic within each device of an IC is not known.
When an IC that contains a device is turned on it is desirable for the device to be in a known state in order to prevent logical uncertainties and possible damage to the device and the IC. A power-on reset (POR) circuit is a device that solves the above mentioned problems. A POR circuit detects when the power supply voltage signal is at an acceptable voltage level. An acceptable voltage level is the voltage at which the binary values of the internal logic can be accurately predicted. The POR circuit then generates a notification signal that is detected by other IC devices. The notification signal notifies the IC devices that the power supply is generating a supply voltage at an acceptable level.
Conventional POR circuits are designed for devices operating at a particular operating voltage. Such POR circuits detect when the power supply voltage signal has reached the acceptable voltage level for its operating voltage. For example, if an IC has an operating voltage of 3.3 volts, a notification signal may be generated by a POR circuit designed to operate with such an IC when the supply voltage is 2.1 volts. Alternatively, if an IC has an operating voltage of 2.0 volts, a notification signal may be generated by a POR circuit designed to operate with such an IC when the supply voltage is approximately 1.25 volts.
A drawback of conventional POR circuits is that they are limited to working with devices operating only at one particular operating voltage. Accordingly, what is needed is a voltage detection circuit, e.g., a POR circuit, that can operate with devices having different operating voltages.